The present invention relates, first, to a passivation material for an electric component according to the preamble of claim 1 and to a piezoelectric component in multilayer construction according to the preamble of claim 11.
Piezoelectric components can, for example, take the form of piezoelectric actuators as multilayer elements with a number of alternately arranged piezoelectric ceramic layers and electrode layers; these are becoming increasingly important in modern electrical engineering. For example, piezoelectric actuators are used as servomechanisms in combination with valves and the like.
A known piezoelectric actuator is, for example, described in detail in DE 196 46 676 C1. In such piezoelectric ceramics, the effect that is put to use is their tendency to develop a charge under mechanical pressure or tension and, conversely, to expand along the main axis of the ceramic layer with the application of an electrical voltage. In order to multiply the useful length elongation, for example, monolithic multilayer actuators are used, consisting of a sintered stack of thin piezoceramic foils (for example, lead zirconate titanate) with inserted metal electrode layers. The electrode layers alternately extend out of the stack and are electrically connected in parallel via external metallization. A striated or tape-like continuous external metallization is applied to both contact sides of the stack, which is connected to all electrode layers having the same polarity.
Between the external metallization and the electrical connections of the piezoelectric actuator, a transfer contact available in a wide variety of forms, for example a Cu-backed Kapton foil strip or similar, is frequently applied. If one applies an electrical voltage to the external contacts, the piezoelectric foils expand in the direction of the field. As a result of the mechanical series connection of the individual piezoelectric foils, the nominal expansion of the entire stack is already achieved at relatively low electrical voltages.
To achieve an optimum deflection of the piezoceramic multilayer actuator with minimum space requirement, the electrode layers must comprise the entire cross section of the actuator, up to, for example, an inactive bonding area. That means that the electrode layers of alternating polarity will be exposed on the lateral surfaces of the multilayer ceramic.
For this reason it is necessary to provide these bare electrode layers with a suitable passivation. In general, a passivation is a type of protective layer, insulation layer or the like used to avoid electric arcing and short circuits between adjacent electrode layers. Without a passivation, such arcing and short circuits could be created, for example, due to mechanical damage to the surface or contamination with dirt (e.g., sanding dust, fingerprints and the like), moisture (e.g., Ag migration or the like), or fuels (for example, diesel fuel, methyl ester of rape seed and similar). Such damage and/or contamination can, in particular, be caused when operating the actuator but also during the manufacturing process.
Passivations are currently manufactured by brushing on a passivation material, which is in the form of a self-adhesive silicone compound, onto the multilayer ceramic by hand after the exterior metallization. This silicone elastomer is then hardened in a drying chamber at 150° C. for about 30 minutes. Then the passivated actuators are sheathed in a cylindrical plastic casing using the same silicone elastomer. The passivation material is selected using the following criteria: First, the passivation material must demonstrate a permanent temperature resistance of >150° C. This is due to the fact that piezoceramic multilayer actuators of the referenced type, for example, are used as actuators in a diesel generator that operates at a similarly high temperature. Also, the passivation material must self-adhere as much as possible to the multilayer ceramic. Finally, up until now it has been a requirement for passivation materials to demonstrate an elongation at break of >200% in order to be able to withstand elongations appearing at the poles during actuator operation.
However, passivation materials using self-adhesive silicone compounds, particularly in the operation of piezoelectric actuators in diesel injectors, have a serious disadvantage. For example, previously used silicone-base passivation materials have limited resistance against swelling when moistened with diesel fuel or methyl ester of rapeseed. Also, the passivation is liable to separate from its surface(s) during operation of the piezoelectric component.